Rocks & Minerals — Set 3

Correct Answer: A. Halite (Rock Salt)

• **Halite (Rock Salt)** = a chemical sedimentary rock (evaporite) formed when saline water evaporates, leaving behind sodium chloride (NaCl) crystals. • **Evaporite deposit** — halite forms in dried lake beds (e.g., Death Valley) or restricted marine basins where evaporation exceeds freshwater input. • 💡 Wrong-option analysis: Slate: metamorphic rock from shale, not an evaporite; Basalt: extrusive igneous rock from lava, not saline water; Coal: organic sedimentary rock from plant material, not evaporation.

Correct Answer: D. Foliation

• **Foliation** = the parallel arrangement of minerals in metamorphic rocks caused by directional pressure, creating distinct layering or banding. • **Foliated examples** — Slate, Schist, and Gneiss all show foliation; non-foliated metamorphic rocks like Marble and Quartzite lack this parallel mineral alignment. • 💡 Wrong-option analysis: Stratification: parallel layering in sedimentary rocks, not metamorphic; Crystallization: formation of crystals from a melt, not the mineral alignment pattern; Lithification: compaction and cementation process forming sedimentary rocks.

Correct Answer: D. Gneiss

• **Gneiss** = a high-grade metamorphic rock with distinctive alternating light (feldspar/quartz) and dark (mica/amphibole) mineral bands, formed under intense heat and pressure. • **High-grade metamorphism** — gneiss forms at temperatures above 600°C and high pressures, often from granite or schist; its banding distinguishes it from other metamorphic rocks. • 💡 Wrong-option analysis: Slate: fine-grained, dark, splits into flat sheets, no banding; Quartzite: white/gray, non-foliated, no distinct bands; Marble: white/colored, non-foliated, no alternating bands.

Correct Answer: A. Organic in origin

• **Organic origin is NOT a mineral requirement** = minerals must be inorganic (not formed from living organisms), naturally occurring, solid, with a crystal structure and definite chemical composition. • **Five mineral criteria** — (1) Naturally occurring, (2) Inorganic, (3) Solid, (4) Crystal structure, (5) Definite chemical composition; coal fails criterion 2. • 💡 Wrong-option analysis: Inorganic: required for mineral definition; Definite chemical composition: required — quartz is always SiO2; Naturally occurring: required — lab-made synthetic diamonds are not minerals.

Correct Answer: C. Lithification

• **Lithification** = the process converting loose sediments into solid sedimentary rock through compaction (squeezing out water) and cementation (mineral binding). • **Compaction + Cementation** — two steps of lithification; minerals like calcite or silica fill pore spaces, binding grains together into solid rock. • 💡 Wrong-option analysis: Metamorphism: transformation of existing rock by heat/pressure; Erosion: wearing away and transport of rock material; Weathering: breakdown of rocks by physical or chemical processes.

Correct Answer: A. Mechanical Weathering

• **Mechanical Weathering (Frost Action)** = water freezes in rock cracks, expands by ~9% volume, and physically forces the rock apart without changing its chemical composition. • **9% volume expansion** — water expands when freezing; repeated freeze-thaw cycles progressively widen cracks until rocks fragment (frost wedging). • 💡 Wrong-option analysis: Oxidation: chemical weathering where oxygen reacts with iron to form rust; Metamorphism: transformation of rock by heat/pressure deep underground; Chemical Weathering: changes rock's chemical composition, unlike frost action.

Correct Answer: A. Chemical Weathering

• **Chemical Weathering (Oxidation)** = oxygen reacts with iron in rocks to form iron oxide (rust / Fe2O3), altering the rock's chemical composition and weakening it. • **Iron oxide (Fe2O3)** — rust formation changes rock chemistry unlike mechanical weathering; oxidized rocks become crumbly and weak. • 💡 Wrong-option analysis: Erosion: transport of weathered material by wind, water, or ice; Thermal Expansion: mechanical weathering by heat and cold cycles; Mechanical Weathering: physically breaks rocks without chemical change.

Correct Answer: A. Limestone

• **Chalk = soft form of Limestone** = a white, porous, soft limestone composed of microscopic shells (coccolithophores) of ancient marine organisms. • **Coccolithophores** — tiny marine algae whose calcite shells accumulated on the seafloor over millions of years to form chalk deposits like the White Cliffs of Dover. • 💡 Wrong-option analysis: Shale: composed of fine clay minerals, dark colored, splits into layers; Coal: organic rock from plant material; Sandstone: composed of sand-sized quartz grains.

Correct Answer: B. Rock Cycle

• **Rock Cycle** = the continuous geological process by which igneous, sedimentary, and metamorphic rocks are transformed into one another over millions of years. • **No permanent rock** — all rocks are recycled; igneous rocks weather into sediment, sediment lithifies into sedimentary rock, which can metamorphose, then melt back to magma. • 💡 Wrong-option analysis: Hydrological Cycle: water cycle (evaporation, precipitation, runoff); Tectonic Cycle: movement of tectonic plates, related but different; Carbon Cycle: movement of carbon through atmosphere, biosphere, and lithosphere.

Correct Answer: C. Specific Gravity

• **Specific Gravity** = the ratio of a mineral's density to that of water (density 1 g/cm³), used to identify minerals; a value of 3 means the mineral is 3× heavier than water. • **Identification tool** — geologists use specific gravity to distinguish minerals; gold (~19) is notably heavy, while quartz (2.65) and halite (2.2) are lighter. • 💡 Wrong-option analysis: Cleavage: mineral's tendency to break along flat planes due to crystal structure; Lustre: the way light reflects off a mineral's surface; Hardness: resistance to scratching, measured on the Mohs scale.